Magnetic random access memory (MRAM) devices are non-volatile memory devices, in which information may be stored based on a magnetoresistive effect of an electrically conductive material therein. In particular, the resistance of the electrically conductive material may be changed depending on an applied magnetic field. MRAM devices may include a plurality of MRAM cells made up of magnetic tunnel junction structures (MTJs) on a single transistor.
The MTJ structure may include multiple thin layers, such as a thin insulating layer sandwiched between two thin ferromagnetic layers. Electrons may tunnel through the thin insulating layer when an external electrical signal is applied. The upper thin magnetic layer is called a free layer, and the lower thin magnetic layer is called a pinned layer.
When the magnetization directions of the free layer and the pinned layer are parallel to each other, a tunneling current flowing through the MTJ may be increased. In other words, the tunneling resistance of the junction may be reduced. In contrast, when the magnetization directions of the free layer and the pinned layer are antiparallel to each other, the tunneling current flowing through the MTJ may be decreased, i.e., the tunneling resistance of the junction may be increased.
MRAM devices may employ magnetic charges to store information, in contrast to conventional memory devices which may employ electrical charges. In other words, digital data (represented by “0” and “1”) may be stored based on a low resistance state (in which the magnetization directions of the two thin magnetic layers are parallel to each other) and a high resistance state (in which the magnetization directions of the two thin magnetic layers are antiparallel to each other).
An anti-ferromagnetic layer, referred to as a pinning layer, may be added to the pinned layer. The pinning layer may serve to fix the magnetization direction of the pinned layer. In particular, the pinned layer may be directly on the pinning layer and may have a relatively large switching field. The magnetization direction of the pinned layer may be fixed in a constant direction when an applied magnetic field is less than the switching field. Thus, the digital data stored in the MRAM cell (i.e., “0” or “1”) may be determined based on the magnetization direction of the free layer. The magnetization direction of the free layer may be changed by the application of a magnetic field. More particularly, to change the magnetization direction of the free layer to a desired direction, conductive interconnections (such as a bit line and/or a digit line) may be formed orthogonal to each other above and below the MTJ. Then as current flows through each conductive interconnection, a magnetic field may be induced by the current.
MTJs may be provided having a rectangular or elliptical shape when seen in a plan view, as magnetic spins in the free layer may be in a more stable state when the magnetic spins are parallel to the longitudinal direction of the free layer.
An MRAM device may include a plurality of MTJs, which may have non-uniform switching characteristics based on their respective fabrication processes. As such, the external magnetic fields for storing desired data in the respective MTJs may be different from one another. Accordingly, the greater the number of MTJs, having non-uniform switching characteristics, the lower the write margin for the MRAM device. In particular, when the MTJs are scaled down to provide higher integration density, the write margin may be significantly reduced. In other words, during a write operation for storing desired data in a selected one of the MTJs, undesired data may be written to other non-selected MTJs that share the bit line and/or the digit line electrically connected to the selected MTJ. As such, with conventional writing methods, write disturbance may occur, and undesired data may be stored in non-selected MTJs during a write operation for storing data in a selected MTJ.
Furthermore, a conventional MRAM cell may have a digit line disposed around the MTJ. Typically, the digit line may be positioned below the MTJ, and the MTJ may have a lower electrode overlapping the digit line. The lower electrode may be electrically connected to a drain region of an access transistor below the digit line. Thus, the lower electrode may extend along a horizontal direction to connect to a contact plug to be formed on the drain region. As a result, an amount by which the planar area of the MRAM cell can be reduced may be limited due to presence of the digit line.
In recent years, MRAM devices have been introduced which employ a spin injection mechanism to address problems related to write disturbance and/or integration density. For example, MRAM devices employing a spin injection mechanism are disclosed in U.S. Pat. No. 6,130,814 to Sun entitled “Current-induced magnetic switching device and memory including the same”. Other MRAM devices employing a spin injection mechanism are disclosed in U.S. Pat. No. 6,603,677 to Redon et al. entitled “Three-Layered Stacked Magnetic Spin Polarization Device with Memory”. The disclosures of U.S. Pat. Nos. 6,130,814 and 6,603,677 are hereby incorporated by reference herein in their entirety.
However, in order to switch the MRAM cell using a spin injection mechanism, a write current density greater than a critical current density may be required. As such, the access transistor may require sufficient drive current capability to provide a write current that is greater than the critical current density. In other words, for MRAM cells employing spin injection mechanisms, scale-down may be limited by current requirements of the access transistors.